Displays capable of electrically rewriting a screen are classified as emissive types that emit light by themselves and non-emissive types that use ambient light for illumination. Thin, lightweight emissive displays include electroluminescence (EL) elements and plasma display panels (PDPs) and non-emissive counterparts include liquid crystal displays. These displays are already available for commercial use.
The liquid crystal display have low contrast, narrow viewing angles, and other shortcomings. A lot of research efforts are put into the development of inorganic and organic EL displays, both belonging to the emissive class of displays, to exploit their properties which readily overcome the disadvantages of liquid crystal displays.
FIG. 25 is a cross-sectional view showing, as an example, the structure of an organic EL display element. In the figure, an organic EL element 1 principally includes an transparent ITO (indium tin oxide) electrode 3, hole transfer layer 4, light-emitting layer 5, electron transfer layer 6, and cathode (for example, aluminum electrode) 7, formed sequentially by, for example, vacuum vapor deposition on a transparent substrate 2 made of glass or other material. Selectively applying a d.c. voltage 8 to the cathode 7 and the transparent electrode 3 acting as an anode moves the holes created in the transparent electrode 3 through the hole transfer layer 4 and the electrons created in the cathode 7 through the electron transfer layer 6 to the light-emitting layer 5 where the electrons and holes are recombined and release light at a predetermined wavelength. An observer can view the produced light L coming out through the transparent substrate 2.
The light-emitting layer 5 may contain, for example, complex compounds of aluminum and zinc. In actual practice, the layer may be made of either an aluminum complex (aluminum complexes are also possible) or a zinc complex (zinc complexes are also possible) with or without additional luminescent material.
The structure entails a problem termed “washout” where intense ambient light in comparison to the light produced by the device, having entered the transparent substrate 2 and been reflected from the cathode 7, produces exceedingly high luminance for a non-emitting state, i.e., black state, reducing contrast greatly and making the screen unrecognizable. The important issue here is to find how to reduce reflected ambient light.
A solution is offered by Japanese Laid-open Patent Application No. 9-127885/1997 (Tokukaihei 9-127885: published on May 16, 1997) disclosing a structure in which circularly polarizing means 11, constituted by a polarizing plate 9 and a quarter-wave plate 10, is disposed in front of the transparent substrate 2 as shown in FIG. 26. With the structure, ambient light incident at the transparent substrate 2 becomes circularly polarized by the polarizing plate 9 and the quarter-wave plate 10. Reflected from the cathode (aluminum electrode) 7 in the organic EL element, the light is still circularly polarized, but oppositely. As the light again passes through the quarter-wave plate 10, it changes its polarization from oppositely circular to parallel to the absorption axis of the polarizing plate 9. The light is therefore absorbed by the polarizing plate 9. Put differently, the ambient light is less reflected. Good contrast thus becomes available. This prevents reflection from mirror-like metal electrodes and loss of contrast.
As to color displays, non-emissive liquid crystal displays are put in practical use. Popularly used among them are transmissive types with a light source in the back, which are enjoying a wide range of applications due to their outstandingly small thickness and light weight when compared to other types of displays. Although only a small amount of energy is needed to modulate the transmittance of a liquid crystal element, the backlight must be continuously powered on no matter what is being displayed; the non-emissive liquid crystal display overall requires large power for its operation. Transmissive color liquid crystal displays are also highly power-consuming, since the backlight emits light in an increased amount to address the problem of low visibility of the display under intense ambient light by boosting the otherwise relatively moderate luminance.
In contrast to these emissive displays and transmissive liquid crystal displays, the reflective liquid crystal display is characterized by its capability to produce display light in an amount proportional to that of ambient light and resultant theoretical immunity to washout. Further, with no need for a backlight, the display can save power for an illumination light source. Conversely, since the reflective liquid crystal display depends on reflected light to produce displays, the displays are not clearly visible under mild ambient light. The problem becomes more evident when color is implemented by the use of a color filter, which absorbs light and further reduces the luminance of the screen.
To enable the use of the reflective liquid crystal display under mild ambient light, a device, typically termed frontlight illumination device, is suggested which is disposed on the screen side of the reflective liquid crystal display to provide illumination. For example, Japanese Laid-open Patent Application No. 11-249132/1999 (Tokukaihei 11-249132; published on Sep. 17, 1999) and No. 11-249133/1999 (Tokukaihei 11-249133; published on the same date) disclose a structure in which an organic EL element is provided as the frontlight on the substrate, of a reflective liquid crystal display, which faces the observer. Meanwhile, Japanese Laid-open Patent Application No. 10-125461/1998 (Tokukaihei 10-125461: published May 15, 1998) discloses a structure in which an organic EL element is provided as the backlight on the back of a liquid crystal display with a metal electrode in the organic EL element doubling as a reflecting plate and another structure in which a transparent organic EL element is provided as an auxiliary light source on a reflective liquid crystal display.
The invention of Tokukaihei 9-127885 restrains development of washout by the provision of the circularly polarizing means. However, the technique is not a result of constructive use of ambient light in display operations: the EL element emits non-polarized light due to the presence of the circularly polarizing plate with about 50% of the light absorbed by that polarizing plate. Results are emission intensity and efficiency which are reduced by about 50%. As would be clear from the description, the element with ambient-light-reflection-preventing means constituted by a polarizing plate and a wave plate has a poor light emission intensity and efficiency which are about half those of the element without the means.
In the inventions as detailed in Tokukaihei 11-249132 and Tokukaihei 11-249133, an organic EL element is located in regions where no display electrodes are located, reducing the effective aperture ratio. On top of it, a cathode made of a magnesium-indium alloy is disposed on the side facing the observer and reflects incoming light back to the observer, reducing contrast. These published applications describe the organic EL element only as a frontlight.
The invention as detailed in Tokukaihei 10-125461 uses the cathode of the organic EL element playing a dual role as a reflecting plate. The structure forms a mirror-like surface and cannot produce a bright white screen with wide viewing angles. Besides, the structure in which the transparent organic EL element is placed in front of the reflective liquid crystal display element allows reflection of ambient light, causing display quality to fall far below satisfactory level. The touch panel, stacked on the display for use, which is disclosed in the application, reflects ambient light and degrades visibility: the unwelcome properties manifest themselves clearly in reflective liquid crystal displays.
The organic EL elements mentioned so far all emit non-polarized light. Research is underway recently for organic EL material emitting polarized light. Emiel Peeters, et al. reported in an article published in J. Am. Chem. Soc. 1997, 119, 9909–9910 that they are conducting research for material that emits circularly polarized light satisfying g≠0, g being given byg=2(IL−IR)/(IL+IR)where IR is the intensity of right circularly polarized light and 1L is the intensity of left circularly polarized light. They, however, mentioned nothing about an optimum device structure to make use of the material.